1. Field of the Invention
This invention relates to an original reading lens and an original reading apparatus using the same, and is particularly suitable for an apparatus for imaging the image information on the surface of an original such as a document, literature, a drawing sheet or film on a reduced scale on the surface of a sensor such as a line sensor and reading it, for example, an image scanner, a film scanner, a digital copying apparatus or the like.
2. Related Background Art
There have heretofore been proposed various original reading apparatuses designed to image the image information on the surface of an original, such as a document or literature, on a reduced scale on the surface of a sensor by an original reading lens, and read the image information from a signal from the sensor as electronic information.
As the original reading lens at this time, a lens system is desired in which the number of constituent lenses is small and the entire lens system is compact and moreover, relatively high optical performance is easily obtained. As an original reading lens satisfying such a desire, there is, for example, a symmetrical type lens system in which a plurality of lenses are disposed substantially symmetrically about a stop. As such symmetrical type lens system, there is, for example, a Tessar type lens of a three-unit, four-lens construction or a Gauss type lens of a four-unit, six-lens construction.
Generally the symmetrical type lens system has the feature that particularly images of high resolving power and high quality can be obtained easily.
When the image information on the surface of an original is to be imaged on a reduced scale on the surface of a sensor by an original reading lens and the image information is to be read as electronic information by a signal from the sensor, it becomes important to image the entire surface of the original on the surface of the sensor with high resolving power.
As an original reading lens, for example, a Tessar type, original reading lens of a three-unit, four-lens construction relatively sufficiently well corrects various aberrations, such as spherical aberration, curvature of the image field, and distortion. In this lens, however, astigmatism at a medium angle of view and coma off the axis have had the tendency to be great and remain. Therefore, the Tessar-type original reading lens is used in an image reading apparatus having low resolution and a narrow angle of view.
Also, this Tessar-type original reading lens is corrected to a certain level with respect to chromatic aberration, particularly on-axis chromatic aberration, but could not always be said to be sufficiently satisfactory as a lens system for a color image reading apparatus.
Particularly, of the on-axis chromatic aberration, the short wavelength side of the visible wavelength region becomes over-corrected and the long wavelength side becomes under-corrected, and in the wide range of the visible wavelength region, it has been difficult to correct well and it had remaining chromatic aberration (secondary spectrum). Therefore, when such an original reading lens is used in a color image reading apparatus, such as an image scanner, there has been the problem that the focus positions in various colors, such as R(red), G(green) and B(blue) somewhat differ from one another and the deterioration of the quality of read image occurs.
On the other hand, as the original reading lens, the Gauss type original reading lens of four-unit, six-lens construction sufficiently corrects various aberrations, such as spherical aberration, coma and curvature of image field. Therefore, it is used in an original reading apparatus having relatively high resolution and a wide angle of view.
However, with respect to chromatic aberration, the Gauss type original reading lens, like the Tessar-type original reading lens, has remaining chromatic aberration (secondary spectrum). Therefore, it has suffered from the problem that the focus positions in the color lights R, G and B differ from one another and the deterioration of the read image occurs.
To correct this remaining chromatic aberration relative to a wavelength of a wide band, it is necessary to use glass having an abnormal partial dispersing property, but there has been the problem that the glass of this kind is generally expensive and difficult to work.